In my case it was the use of
/ in the report name as in
Origin hits/day (-7days). This is a bug in Splunk. If you don't have a
/ look for another character that could be throwing it off for just panel usage.
Thanks for finding this out.
Ran into the same problem and probably would not have figured it out myself.
Interesting variation I had on this: it worked fine when accessing splunk with a short hostname (eg: splunk:8000) and I only got the error when using a fully qualified domain name (eg: splunk.my.domain.org:8000).
Have you checked the xml? I am currently having this same issue and saw that the xml is missing. I have no idea how it got deleted but without that I don't think the search will be able to run. Hope this helps.
your search query